<!doctype html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport"
          content="width=device-width, user-scalable=no, initial-scale=1.0, maximum-scale=1.0, minimum-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>fixed</title>
    <style>
        * {
            padding: 0;
            margin: 0;
        }

        main {
            width: 100vw;
            height: 200vh;
            background-color: cyan;
        }

        div {
            width: 100px;
            height: 100px;
            background-color: white;
            position: relative;
            top: 100px;
            left: 100px;
        }
        article{
            width: 50px;
            height: 50px;
            /* 相对于viewport 适口*/
            position: fixed;
            background-color: purple;
            bottom: 0;
            right: 0;
        }
    </style>
</head>
<body>
<main>
    <div>
        <article></article>
    </div>
</main>
</body>
</html>
